Identification and chromosomal distribution of 5S rRNA genes in Neurospora crassa

The 5S rRNA genes of Neurospora crassa, unlike those of most organisms, are not tandemly arranged, and they are found imbedded in a variety of unique sequences. The 5S rRNA regions of most of the genes are of one type, alpha; however, several other "isotypes" (beta, gamma, delta, zeta, and eta) are also found. We asked whether Neurospora 5S rRNA genes are dispersed on a chromosomal scale and whether genes of different isotypes are spatially segregated. We identified, by DNA sequencing, 5S rRNA genes in 22 5S DNA clones, and we mapped these genes by conventional crosses by using restriction fragment length polymorphisms in their flanking sequences as genetic markers. The results show that the 5S rRNA genes are distributed on at least six of the seven chromosomes. Their location does not appear to be completely random. Some of them are closely linked. One of the chromosomes carries a disproportionate number of 5S rRNA genes of the most common structural type, alpha; another chromosome carries three of the four mapped beta 5S rRNA genes. None of the 5S rRNA genes studied maps close to the nucleolus organizer, the site of the genes that code for the three larger rRNAs.